I. Technical Field
This invention relates to telecommunications, and particularly to the structure of frames transmitted over a wireless or radio interface.
II. Related Art and Other Considerations
In a typical cellular radio system, wireless terminals (also known as mobile terminals, mobile stations, and mobile user equipment units (UEs)) communicate via base stations of a radio access network (RAN) to one or more core networks. The wireless terminals (WT) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network. The base station, e.g., a radio base station (RBS), is in some networks also called “NodeB” or “B node”. The base stations communicate over the air interface (e.g., radio frequencies) with the wireless terminals which are within range of the base stations. The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UTRAN is essentially a radio access network providing wideband code division multiple access for user equipment units (UEs). The radio access network in a UMTS network covers a geographical area which is divided into cells, each cell being served by a base station. Base stations may be connected to other elements in a UMTS type network such as a radio network controller (RNC). The Third Generation Partnership Project (3GPP or “3G”) has undertaken to evolve further the predecessor technologies, e.g., GSM-based and/or second generation (“2G”) radio access network technologies.
The IEEE 802.16 Working Group on Broadband Wireless Access Standards develops formal specifications for the global deployment of broadband Wireless Metropolitan Area Networks. Although the 802.16 family of standards is officially called WirelessMAN, it has been dubbed WiMAX” (from “Worldwide Interoperability for Microwave Access”) by an industry group called the WiMAX Forum.
IEEE 802.16e-2005 (formerly known as IEEE 802.16e) is in the lineage of the specification family and addresses mobility by implementing, e.g., a number of enhancements including better support for Quality of Service and the use of Scalable OFDMA. In general, the 802.16 standards essentially standardize two aspects of the air interface—the physical layer (PHY) and the Media Access Control layer (MAC).
Concerning the physical layer, IEEE 802.16e uses scalable OFDMA to carry data, supporting channel bandwidths of between 1.25 MHz and 20 MHz, with up to 2048 sub-carriers. IEEE 802.16e supports adaptive modulation and coding, so that in conditions of good signal, a highly efficient 64 QAM coding scheme is used, whereas where the signal is poorer, a more robust BPSK coding mechanism is used. In intermediate conditions, 16 QAM and QPSK can also be employed. Other physical layer features include support for Multiple-in Multiple-out (MIMO) antennas in order to provide good performance in NLOS (Non-line-of-sight) environments and Hybrid automatic repeat request (HARQ) for good error correction performance.
In terms of Media Access Control layer (MAC), the IEEE 802.16e encompasses a number of convergence sublayers which describe how wireline technologies such as Ethernet, ATM and IP are encapsulated on the air interface, and how data is classified, etc. It also describes how secure communications are delivered, by using secure key exchange during authentication, and encryption during data transfer. Further features of the MAC layer include power saving mechanisms (using Sleep Mode and Idle Mode) and handover mechanisms.
The IEEE standard 802.16m is intended to be an evolution of IEEE standard 802.16e with the aim of higher data rates and lower latency. There is a requirement for backward compatibility between IEEE standard 802.16m and its IEEE standard 802.16e predecessor. Yet the frame structure of IEEE standard 802.16e poses problems for backward compatibility, as explained below.
The frame structure for IEEE standard 802.16e is shown in FIG. 1. The frame length for IEEE standard 802.16e is 5 ms, and uses time division multiplexing (TDD). The preamble is used by mobile stations to synchronize to the downlink (DL), and the DL-MAP messages that occur just following the preamble give allocation information to the mobile stations on the downlink and the uplink. Examples of downlink and uplink allocations are shown in FIG. 1. The transmit transition gap (TTG) and the receive transition gap (RTG) are gaps used for the mobile station to switch from receive to transmit and vice versa.
In IEEE standard 802.16e, the desired latency for a transmission on the downlink (DL) and a single retransmission is 20 milliseconds. Thus, as illustrated in FIG. 2, a base station can send a data block (transmission) in Frame 1, and the mobile station (MS) can send a negative acknowledgement (NACK) in Frame 2, allowing for processing delay at the mobile station (MS). With processing and scheduling delay at the base station, a retransmission can be sent in Frame 4, which is within the 20 ms delay budget.
For IEEE standard 802.16m, on the other hand, a latency of 10 ms is desirable on the downlink (DL). As shown in FIG. 2B, this means that the retransmission has to be sent in the very next frame after a transmission. This is only possible if the NACK is sent by the mobile station (MS) in the same frame, and the base station reacts immediately to the NACK to send the retransmission. This shorter latency imposes a significant processing burden on the base station (to process, e.g., the NACK and generate the re-transmission) and the mobile station (MS) (to determine if a NACK is necessary and to generate the NACK). In addition, flexibility of allocation may be lost. For example, it may be absolutely necessary for the allocation to the mobile station (MS) to be in the beginning of the first fame so that the mobile station (MS) has sufficient time to react. Also, the downlink (DL)-uplink (UL) ratio (DL:UL) cannot be too high, otherwise the base station will have too little time to process the NACK and to send the retransmission.
There is also an additional requirement or desire in 802.16m to enhance the throughput of the system at high vehicular speeds. Wireless data systems typically use a process of link adaptation to adjust the transmit signal format to a mobile station based on its instantaneous channel conditions in order to optimize the throughput of the wireless link. Such link adaptation is typically based on feedback of channel quality information from the MS, which is typically sent once per frame. With high vehicular speeds, the channel changes more rapidly, and the feedback information sent from the MS may be outdated.
It would be advantageous to have a shorter frame length in order to meet the desired latency for IEEE standard 802.16m, while at the same time preserving the flexibility of allocation. The shorter frame length would also allow the feedback of channel quality information at a higher rates, thereby facilitating the improvement of data throughputs at high vehicular speeds. However, legacy mobile stations (including, for example, mobile stations subscribing to IEEE standard 802.16e) may not be able to operate with a smaller frame length.